Herpes simplex virus (HSV) keratitis is the leading cause of non-traumatic blindness in the U.S., with approximately 500,000 cases diagnosed each year. HSV can also cause cold sores, genital lesions, and encephalitis. HSV infections are resistant to cure due to the virus's unique ability to establish lifelong latent infections within sensory neurons. Following entry into its host, HSV replicates at peripheral sites such as the eyes, skin, or mucosae. It then gains access to the distal axonal terminae of sensory neurons and travels by intra-axonal transport to neuronal cell bodies in sensory ganglia, where further viral replication may occur. Viral gene expression is subsequently repressed and latency is established. No free infectious virus can be detected during latency, although the virus remains transcriptionally active, producing the latency associated transcripts (LATs). The precise function(s) of these LATs are unclear but they appear to play a role in promoting viral reactivation from latency. Latency therefore represents a lifelong source of virus which can reactivate periodically causing severe ocular and other mucocutaneous tissue damage. The mechanisms involved in the virus's unique ability to switch between latency and reactivation are poorly understood. The broad objectives of this proposal are therefore to achieve a better understanding of the mechanisms involved in this switch at the molecular level. This proposal aims to define more precisely regions in the viral genome that are critical for its responsiveness to cellular trans-acting factors and to investigate the roles of viral and cellular factors in the establishment, maintenance, and reactivation of latency. To this end, mutations will be generated in cloned copies of HSV latency-related genes and the effects of these mutations evaluated by a series of in vitro transcription and DNA-binding assays. Selected mutations will then be introduced into the viral genome and recombinant viruses will be tested in a mouse ocular latency model. This approach will assess the effects of such mutations upon the pathogenesis of the virus, its ability to establish latency and its ability to reactivate in response to defined stimuli. A better understanding of the mechanisms and stimuli involved in viral reactivation may indicate new antiviral targets and enable the development of novel therapeutic approaches and agents for interrupting the viral lifecycle at this pivotal stage, allowing control of this blinding disease.